The air traffic control radar beacon system (“ATCRBS”) was developed in 1956 as a secondary surveillance radar system for use within an air traffic control system for more accurate location of aircrafts. In the ATCRBS, a ground station transmits signals, known as “interrogations” to all aircraft within the ground station's range. There are two types of ATCRBS interrogations: Mode A interrogations and Mode C interrogations. Mode A interrogations represent a request for the identifier of the interrogated aircraft. Mode C interrogations represent a request for the altitude of the aircraft. FIG. 1 illustrates a Mode A interrogation. Mode A interrogations comprise a first pulse, P1, and a second pulse, P2, separated by 2 μs, and a third pulse P3, separated by 8 μs from the P1 pulse. FIG. 2 illustrates a Mode C interrogation. Mode C interrogations comprise a first pulse, P1, a second pulse P2, separated by 2 μs from the first pulse P1, and a third pulse, P3, separated by 21 μs from the P1 pulse. Both P1 and P3 pulses are transmitted by the main lobe of a directional antenna. In addition, in both Mode A and Mode C interrogations, a second pulse, P2, is sent 2 μs after the first pulse, P1, using an omni-directional antenna in order to prevent the transponder from responding to interrogation signals that do not originate from the main lobe of the directional antenna. If P2 is equal to or greater than P1, the aircraft is not in the main lobe of the directional antenna and the transponder suppresses its response to the interrogation.
Upon receipt of the interrogation signal, the aircraft develops a reply signal to supply the requested information consisting of identification and/or altitude location depending on the interrogation mode. The ground station processes this information, together with time of arrival range information, to develop a measurement of position for each responding aircraft. In an ATCRBS environment, each aircraft includes a transponder that receives interrogations from and sends replies to a ground station.
There are several drawbacks to the ATCRBS system. For example, the interrogated aircraft can only provide an identifier and an altitude. The Mode C and Mode A systems also encounter a large amount of interference and garble because the Mode C and Mode A interrogation from an ATCRBS beacon interrogation is special, i.e., all aircraft transponders within the main beam of the interrogating ground station reply. This means that 25-45 interrogations and replies are transmitted into the radio frequency environment. This results in proliferation of radio frequency transmissions which can result in a significant amount of interference or garble and a corresponding loss of integrity.
In recognizing these and other deficiencies in the ATCRBS, the Mode Select (Mode S) system was developed to allow the active transmission of information by a ground station or another aircraft which substantially reduced transmission interference and garble. The Mode S beacon system allows for the addressing of specific aircrafts and thereby decreases interference. In a Mode S system, an interrogation is sent to all aircrafts in the ground station's range. This interrogation is known as an “All Call” interrogation FIG. 3 illustrates a ATCRBS/Mode S “All Call” interrogation. Pulses P1, P2, P3 and P4 are transmitted by the main lobe of the ground station's antenna. A similar pattern can be used to send an ATCRBS “All Call” interrogation. The difference between the ATCRBS “All Call” interrogation and the ATCRBS/Mode S “All Call” interrogation is the width of the P4 pulse. For an ATCRBS “All Call” interrogation, the P4 pulse is 0.8 μs in width, while a ATCRBS/Mode S “All Call” interrogation is a 1.6 μs P4 pulse.
A second type of Mode S interrogation is illustrated in FIG. 4. The interrogation comprises pulse P1 and pulse P2 followed by a pulse P6 which can be a data-containing pulse of either 16.25 μs or 30.25 μs. The pulse P2 is sent with an amplitude equal to or greater than P1 in order to suppress ATCRBS responses. As can be appreciated, determining whether a pulse has been received as well as determining the magnitude and/or relative magnitude of the pulse to distinguish between ATCRBS and Mode S transmissions is important.
In view of the foregoing, it is desirable to provide a method for real time pulse processing in ATCRBS/Mode S transponders that addresses one or more of the foregoing deficiencies or other deficiencies not implicitly or expressly described. Furthermore, other desirable factors and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.